The present invention relates to magnetic recording media, such as thin film magnetic recording disks, to a method of manufacturing the media and to an apparatus employed to manufacture the media. The present invention has particular applicability to high areal density longitudinal magnetic recording media exhibiting low media noise and enhanced magnetic performance.
Magnetic recording media are extensively employed in the computer industry and can be locally magnetized by a write transducer or write head to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based upon bits of the information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium, grains of the recording medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium can subsequently produce an electrical response to a read sensor, allowing the stored information to be read.
There is an ever increasing demand for magnetic recording media with higher storage capacity, lower noise and lower costs. Efforts, therefore, have been made to reduce the size required to magnetically record bits of information, while maintaining the integrity of the information as size is decreased. The space necessary to record information in magnetic recording media depends upon the size of transitions between oppositely magnetized areas. It is, therefore, desirable to produce magnetic recording media that will support the smallest transition size possible. However, the output from small transition sizes must avoid excessive noise to reliably maintain the integrity of the stored information. Media noise is generally expressed in signal-to-noise ratio (SMNR).
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), medium noise, i.e., SMNR, and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the Hr of the magnetic recording medium. This objective must be accompanied by a decrease in the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise in thin films is a dominant factor restricting increased recording density of high density magnetic hard disk drives, and is attributed primarily to big and inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
Longitudinal magnetic recording media containing cobalt (Co) or Co-based alloy magnetic films with a chromium (Cr) or Cr alloy underlayer deposited on a non-magnetic substrate have become the industry standard. For thin film longitudinal magnetic recording media, the desired crystallized structure of the Co and Co alloys is hexagonal close packed (hcp) with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis is in the plane of the film. The better the in-plane c-axis crystallographic texture, the more suitable is the Co alloy thin film for use in longitudinal recording to achieve high remanance. For very small grain sizes coercivity increases with increased grain size. The large grains, however, result in greater noise. Accordingly, there is a need to achieve high coercivities without the increase in noise associated with large grains. In order to achieve low noise magnetic recording media, the Co alloy thin film should have uniform small grains with grain boundaries capable of magnetically isolating neighboring grains. This type of microstructural and crystallographic control is typically attempted by manipulating the deposition process, and proper use of underlayers and seedlayers.
Underlayers can strongly influence the crystallographic orientation, grain size and chemical segregation of the Co alloy grain boundaries. Conventional underlayers include Cr and alloys of Cr with elements such as titanium (Ti), tungsten (W), molybdenum (Mo) and vanadium (V).
There are other basic characteristics of magnetic recording media, aside from SMNR, that are indicative of recording performance, such as half-amplitude pulse width (PW50), overwrite (OW), and modulation level. A wide PW50 indicates that readback pulse from adjacent bits are crowded together resulting in interference which limits the linear packing density of bits in a given track and, hence, reduces packing density in a given area thereby limiting the recording capacity of the magnetic recording medium. Accordingly, a narrow PW50 is desirable for high areal recording density.
OW is a measure of the ability of the magnetic recording medium to accommodate overwriting of existing data. In other words, OW is a measure of what remains of a first signal after a second signal, e.g., at a different frequency, has been written over it on the medium. OW is considered low or poor when a significant amount of the first signal remains.
It is extremely difficult to obtain optimum performance from a magnetic recording medium by optimizing each of the PW50, OW, SMNR and modulation level, as these performance criteria are interrelated and tend to be offsetting. For example, if coercivity is increased to obtain a narrower PW50, OW is typically adversely impacted, as increasing coercivity typically degrades OW. A thinner medium having a lower Mrxc3x97thickness (Mrt) yields a narrower PW50 and better OW; however, SMNR decreases since the medium signal is typically reduced if the electronic noise of the system is high. Increasing the squareness of the hysteresis loop contributes to a narrower PW50 and better OW; however, noise may increase due to intergranular exchange coupling and magnetostatic interaction. Thus, a formidable challenge is present in optimizing magnetic performance in terms of PW50, OR, SMNR and modulation level.
It is recognized that the magnetic properties, such as Hr, Mr, S* and SMNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by the underlying layers, such as the underlayer. It is also recognized that underlayers having a fine grain structure are highly desirable, particular for growing fine grains of hcp Co alloys deposited thereon.
The majority of current conventional longitudinal magnetic recording media exhibit a bi-crystal cluster structure comprising a Co alloy with a (1.0) texture or crystallographic orientation epitaxially grown on a Cr-containing underlayer exhibiting a (200) texture. A bi-crystal cluster structure is characterized by two Co subgrains with an easy axis perpendicular to each other.
It has been reported that nickel-aluminum (NiAl) films exhibit a grain size which is smaller than similarly deposited Cr films, and can be employed to produce magnetic recording media with a Co-alloy having a (10.0) texture grown on the underlayer with a (112) texture forming the so called xe2x80x9cuni-crystalxe2x80x9d structure. Li-Lien Lee et al., xe2x80x9cNiAl Underlayers For CoCrTa Magnetic Thin Filmsxe2x80x9d, IEEE Transactions on Magnetics, Vol. 30, No. 6, November, 1994, pp. 3951-3953, and U.S. Pat. No. 5,693,426 issued to Li-Lien Lee et al. The xe2x80x9cuni-crystalxe2x80x9d structure is characterized by Co grains having an easy axis randomly distributed in the film plane. Accordingly, NiAl thin films are potential candidates as underlayers for magnetic recording media for high density longitudinal magnetic recording. However, it was found that the coercivity of a magnetic recording medium comprising an NiAl underlayer is too low for high density recording, e.g. about 2,000 Oersteds (Oe). The use of a NiAl underlayer is also disclosed by C. A. Ross et al., xe2x80x9cThe Role Of An NiAl Underlayers In Longitudinal Thin Film Mediaxe2x80x9d, J. Appl. Phys. 81(8), P.4369, 1997. NiAl underlayers are also disclosed by Lee et al. in U.S. Pat. No. 5,693,426 and Lee et al. in U.S. Pat. No. 5,800,931. A magnetic recording medium comprising a NiAl seedlayer under a Cr underlayer is disclosed by Zhang in U.S. Pat. No. 5,858,566.
The effective crystal anisotropy constant of the bi-crystal cluster structure media is less than the intrinsic crystal anisotropy constant, leading to a lower coercivity than that of uni-crystal media, Qingzhi Peng et al., xe2x80x9cMicromagnetic and Experimental Studies of CoPtCr Polycrystalline Thin Film Media with Bicrystal Microstructurexe2x80x9d, IEEE Transactions on Magnetics, Vol. 31, No. 6, November, 1995, pages 2821-2823. With increasing recording density, higher coercivity is required.
Cobalt-chromium-platinum-tantalum-niobium (CoCrPtTaNb) magnetic alloys had been reported. See, for example, H. Akimoto et al., xe2x80x9cMagnetic Interaction in Coxe2x80x94Crxe2x80x94Ptxe2x80x94Taxe2x80x94Nb Media; Utilization of Micromagnetic Simulationxe2x80x9d, IEEE Transactions on Magnetics, Vol. 34 No. 4, July 1998, pages 1597-1599. CoCrPtTaNb and CoCrPtTa magnetic alloys with a high-Cr concentration, e.g., about 16 to about 21 at.%, are suitable candidates for high density magnetic recording media due to their high coercivity and low noise performances.
However, media containing such high-Cr CoCrPtTaNb or CoCrPtTa magnetic alloys deposited on Cr-containing underlayers on a NiAl seedlayer do not develop the desired uni-crystal structure employing an in-line pass-by sputtering system, particularly when the Cr-containing underlayer is about 100 xc3x85 or less in order to preserve a small underlayer grain size.
In U.S. Pat. No. 5,762,071 issued to Chen et al., magnetic recording media are disclosed comprising a dual magnetic layer of CoCrTa and CoCrPtTa on a Cr-containing underlayer for improved magnetic performance of a bi-crystal cluster media. Alex, in U.S. Pat. No. 5,616,218 discloses a sputtering system comprising a collimator for varying the deposition rate, arrival energy of particles and angular distribution of particles to affect the crystal texture of an underlayer in manufacturing a magnetic recording medium. Zhang, in U.S. Pat. No. 5,772,857, discloses a magnetic recording medium comprising dual magnetic films of CoCrTaPt on CoCrTa. Ohkubo, in U.S. Pat. No. 5,851,656, discloses a magnetic recording media comprising multiple magnetic layers with an intermediate non-magnetic Cr alloy layer.
There exists a need for high areal density longitudinal magnetic recording media exhibiting high Hr, high SMNR and high in-plane coercivity. There exists a particular need for magnetic recording media containing a high-Cr CoCrPtTaNb or CoCrPtTa magnetic alloy and exhibiting a uni-crystal structure with high in-plane coercivity.
An advantage of the present invention is a magnetic recording medium for high areal recording density exhibiting low noise, and high in-plane coercivity.
Another advantage of the present invention is a magnetic recording medium for high areal recording density containing a high-Cr CoCrPtTaNb or CoCrPtTa magnetic alloy and exhibiting a uni-crystal structure with high in-plane coercivity.
A further advantage of the present invention is an apparatus for manufacturing a magnetic recording medium for high areal recording density with a uni-crystal structure exhibiting low noise and high in-plane coercivity containing a high-Cr CoCrPtTaNb or CoCrPtTa magnetic alloy.
Another advantage of the present invention is a method of manufacturing a magnetic recording medium suitable for high areal recording density with a uni-crystal structure exhibiting low noise and high in-plane coercivity comprising a high-Cr CoCrPtTaNb or CoCrPtTa magnetic alloy.
Additional advantages and features of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following only to be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained and particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved by a magnetic recording medium comprising: a non-magnetic substrate; a nickel-aluminum (NiAl) seedlayer on the substrate, a chromium (Cr) or Cr alloy underlayer on the seedlayer; a first cobalt chromium tantalum (CoCrTa) magnetic layer on the underlayer and a second magnetic layer on the first CoCrTa magnetic layer.
Another aspect of the present invention is a method of manufacturing a magnetic recording medium, the method comprising: depositing a NiAl seedlayer on a non-magnetic substrate; depositing a Cr or Cr alloy underlayer on the NiAl seedlayer; depositing a first CoCrTa magnetic layer on the underlayer and depositing a second magnetic layer on the first CoCrTa magnetic layer.
A further aspect of the present invention is an apparatus for manufacturing a magnetic recording medium, the apparatus comprising: a conveyor for moving a non-magnetic substrate by a plurality of sputter deposition chambers; a chamber for sputter depositing a NiAl seedlayer on the non-magnetic substrate; a chamber for sputter depositing a Cr or Cr alloy underlayer on the seedlayer; a chamber for sputter depositing a first CoCrTa magnetic layer on the underlayer; and a chamber for sputter depositing a second magnetic layer on the first CoCrTa magnetic layer.
Embodiments of the present invention comprise depositing a CoCrPtTaNb or CoCrPtTa magnetic alloy containing about 16 to about 21 at.% Cr on the first CoCrTa magnetic layer. Embodiments of the present invention further comprise sputter depositing the first CoCrTa magnetic alloy in an in-line pass-by sputtering apparatus containing a sputtering chamber with a shield positioned such that the minimum angle of incidence of impinging atoms is no less than about 26xc2x0, whereby the ratio of perpendicular coercivity over in-plane coercivity of the magnetic recording medium is less than about 0.5, e.g., less than about 0.4. The CoCrTa magnetic alloy layer is typically deposited at a thickness of about 2 xc3x85 to about 50 xc3x85 and enhances the (10.0) crystallographic orientation of the second magnetic layer such that the second magnetic layer exhibits a predominant uni-crystal structure.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.